The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In wireless sensor networks, sensors are responsible for detecting, monitoring, and tracking events with as low an energy consumption as possible. A specific application of sensor networks is binary event sensing. An example application for such sensing is structural health monitoring. For example, a sensor network may monitor the structural integrity of a bridge. In various implementations, each sensor may simply determine whether or not the presence of an abnormality, such as a crack, has been sensed. The sensor may be configured to send a fatigue event notice in response to sensing the abnormality and remain silent otherwise.
In another example, a sensor network may perform intrusion detection. Each sensor may transmit an intrusion event notice in response to sensing intrusion in its vicinity. A sink receives transmitted event notices. The sink, or a further processing system, can analyze the timing and approximate location of multiple binary failure events or intrusion events to study the dynamics of structural failure or the path of an intruding entity.
Simply transmitting an indication of the occurrence of an event requires a single bit of information (i.e., binary information). Traditional packet communication is highly energy inefficient when transmitting a single bit of actual data. Such inefficiency stems from communication, processing, and buffering overhead for each packet (including header, checksum, and synchronization preamble). Instead of incurring the overhead of a packet, sensors may transmit a single pulse to signify the occurrence of an event. In a multi-hop network (i.e., where one or more sensors cannot transmit directly to the sink), sensors may relay pulses from other sensors toward the sink.
Hop-Angular
A pulse generated by a sensor represents the occurrence of the event as well as its location of origin. A concept of hop-angular event area can be used for event localization. The network is logically divided into a fixed number of angular sectors. In FIG. 1, for example, there are 16 22.5°-wide sectors. Given a pre-defined sector width (22.5° in FIG. 1), the location of a sensor can be represented by the tuple [sector_id, hop_distance], where hop_distance is the number of transmissions required for a pulse to arrive at the sink. Meanwhile, if the sectors are numbered counter clockwise, with the first sector above 0° being sector 1, the location of the hashed Event Area in FIG. 1 can be represented as [15, 3]. This means the node is located in the 15th sector and has a hop_distance of 3 from the sink. This tuple for an event's origin is carried to the sink by the corresponding pulse.
The concept of event area does not assume any specific shape (i.e., circular or otherwise) of a node's transmission coverage area. It could be of any arbitrary shape as shown in FIG. 1. While the angle for a node is pre-programmed at deployment time, its hop distance may be dynamically discovered. The worst-case resolution for event localization increases with increasing sector width and increasing transmission range.
Frame Structure
Nodes are time-synchronized by the sink at the beginning of each frame, which defines the times during which the sink and the sensors can transmit. See FIG. 2 for the frame definition. Each slot can be used for sending a single pulse. The slot includes a guard time so that even with the maximum amount of clock drift, a pulse transmission will still occur within the desired slot.
As shown in FIG. 2, the frame contains an uplink (toward the sink) portion and a downlink (away from the sink) portion. The uplink portion includes a control sub-frame and an event sub-frame. The downlink portion of the frame includes a synchronization slot in which the sink transmits a full-power pulse to all the nodes for synchronizing the clocks. The two following downlink slots of the downlink portion and the reconfiguration portion of the uplink control sub-frame are used for hop-distance discovery. The reconfiguration area in the control sub-frame has (H+1) slots, where H is the maximum hop-distance. The forwarding flag area is designed for routing pulses toward the sink. The H-slot-wide routing area of the control sub-frame is used for energy management.
The event sub-frame contains H slot clusters, with each cluster containing 360/α slots, where α is the sector width. Each of the slots therefore corresponds to a different specific [sector-id, hop-distance] tuple. An event-originating node transmits a pulse during the dedicated event sub-frame slot that corresponds to the [sector-id, hop-distance] of the node's event area.
Pulse Forwarding
While routing the pulse toward the sink, at each intermediate node, the pulse is transmitted in the same event sub-frame slot that corresponds to the [sector-id, hop-distance] of its event area of origin. In other words, while being forwarded, the transmission slot for the pulse at all intermediate nodes does not change with respect to the frame. This is how information about the location of origin of an event is preserved during routing. Upon reception, the sink can infer the event area of origin from the [sector-id, hop-distance] value corresponding to the slot during which the pulse is received.
A hop-distance discovery process may be periodically executed by the network for each node to discover its own hop-distance from the sink node. When a pulse is transmitted by a node at hop-distance h, only neighboring nodes at hop area (h−1) forward the pulse toward the sink. In other words, the nodes at hop area h and (h+1) should ignore the pulse. This logic ensures that a pulse is eventually delivered to the sink.
While transmitting a pulse in the event sub-frame, the sensor also transmits a pulse in the corresponding slot of the forwarding flag area of the control sub-frame. That is, while forwarding a pulse from a hop area h node, a pulse is also transmitted in the hth time slot of the forwarding flag area. By looking at the received pulse in the forwarding area, all the receivers of the pulse can decide if it should be discarded or forwarded toward the sink. This can protect against detection of a false pulse and also ensure that a pulse from hop-area h is forwarded only by nodes in hop-area (h−1).
For additional information regarding hop-angular routing, see Q. Huo and S. Biswas, “A Novel Concept of UWB Pulse Switching in Sensor Networks,” ICWMC 2012 and Q. Huo, A. Plummer, and S. Biswas, “Pulse Switching for Static Event Sensing in Sensors Networks,” IEEE Globecomm, December 2011, the entire disclosures of which are hereby incorporated by reference into this application.
In the drawings, reference numbers may be reused to identify similar and/or identical elements.